[Feature request] Add config option to gpgsign IFF key is present

Currently, `commit.gpgsign` allows you to give either 'true' or 'false' as a value. If the key is not present, commits will fail:

```sh $ git commit -m "example" error: gpg failed to sign the data fatal: failed to write commit object ```

I like to reuse my config file across several machines, some of which do not have my GPG key. Would it be possible to add an option to sign the commit only if the private key for `user.signingkey` is present? It could be named something like `commit.gpgsign=default-yes`.

Thank you for your time, Joshua Nelson